Breeder male Sprague-Dawley rats were categorized as potential low and high protein excretors at 4 months of age. The average selection values of these two groups were 0.59 and 1.75 mg/ml, respectively. Paris of females were mated to these males in two breeding cycles in the fashion of a diallele cross. The heritability index based on protein excretion values of 263 male offspring at 6 months of age from both cycles of high and low excretor males was 0.526 with approximate variance of 0.0668. Significantly different (p less than 0.01) survival distributions were shown. The mean and median survival times of the high excretor offspring were 611.1 and 652 days, respectively, compared to 651.4 and 680.5 days, respectively, in low excretor offspring. The proportion of offspring with marked chronic progressive nephrosis was significantly greater (p less than 0.01) in the high excretor group, both prior to and after 700 days of age, than that of offspring in the low excretor group. The selection of low excretor males for breeding stock appears to offer a practical basis for minimizing the total expression of chronic progressive nephrosis in this strain of rat.